Field of the Invention
The invention relates to a guide tube which extends into a pressure vessel, in particular into a pressure vessel in a nuclear power station, and in which a so-called instrumentation lance is guided. The invention also relates to a method for preventing an accumulation of particles, in particular, radioactively loaded particles in the guide tube outside the pressure vessel.
The guide tube is often passed into the pressure vessel through its base, so that it includes an upper tube part, which is arranged in the pressure vessel, and a lower tube part, which projects out of the pressure vessel. The lower tube part is closed at the end by a closure flange. The guide tube has a flow connection to the interior of the pressure vessel and is also filled with water, in the same way as the pressure vessel. The pressure vessel is, in particular, a reactor pressure vessel or else a steam generator for a nuclear power station, in which radioactively loaded water or steam is located. The instrumentation lance normally has measured instrumentation for measuring the pressure, the temperature, the neutron flux, the filling level, etc. in the pressure vessel.
Temperature fluctuations result in the water being forced into the interior of the pressure vessel out of the guide tube when it is heated. Conversely, when the temperature decreases, water is sucked out of the pressure vessel into the guide tube. The temperature changes are caused, for example, by different operating states of the reactor pressure vessel. Large temperature differences occur in particular when starting and shutting down the nuclear power station, during which, for example, the water that is located in the reactor pressure vessel is heated from 20° to 300° C.
When water enters the guide tube from the pressure vessel, radioactively loaded contamination particles, inter alia, are also introduced into the guide tube, and are deposited at the lower end of the guide tube, which is closed by the closure flange. There, they form a highly radioactively emitting source outside the pressure vessel and represent a significant danger source for the workers during maintenance work.